MIPS: Porting r10221 to ARM (avoid bailing out to runtime for short substrings).

Port r10250 (77889cc).

BUG=
TEST=

Review URL: http://codereview.chromium.org/8953013
Patch from Daniel Kalmar <kalmard@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10264 ce2b1a6d-e550-0410-aec6-3dcde31c8c00